based on your question I am assuming that you are buying a bank-owned (REO) property? You accepted their counter-offer to you and it may have been a multiple counter offer? If so, that could be the case. My best advice to you is to keep in close contact with your agent. I'm sure he or she will advise you if they feel there is anything else you could do to make your offer stronger.
But, yes it is an open playing field until both parties have signed a contract. Good luck to you, keep us informed as to the outcome.
You didn't "ACCEPT" any counter offer unless it's signed by the seller. If the seller didn't sign the paperwork, what you have done is let the seller tell you what to offer. IF there is no signature there is no deal, period.
That being said, this is stanard operating procedure for most banks.
a contract has to have all relevant signatures on it to be binding. Did you have an return of acceptence time on your contract? Your agent should be able to tell you.